1. Field of the Invention
The present invention relates to a micro-structure which can be produced by micromechanical processing.
2. Related Art
In recent years, micro-machines having small movable parts have been investigated. In particular, in the case of making microstructures by using technologies for semiconductor integrated circuits, such as those which include photolithographic processing, micro-parts can be reproduced accurately. Thereby, the parts can be arrayed easily on a substrate and the parts can be produced at low cost. The parts can respond quicker than parts produced by prior techniques because of their reduced size.
Three typical methods of producing a micro-structure on a substrate are described below.
One is a process for producing a micro-motor (M. Mehregany et al, "Operation of micro fabricated harmonic and ordinary side-drive motors"--Proceedings IEEE Micro Electro-Mechanical Systems Workshop 1990, pp. 1-8) or a linear microactuator (P. Chenung et al, "Modelling and position-detection of a polysilicon linear microactuator", Micro mechanical Sensors, Actuators and Systems ASME 1991, DS C-Vol.32, pp. 269-278).
This first process comprises: forming a silicon dioxide layer and a polysilicon layer on a silicon substrate in this order, or providing a SOI (silicon on insulator) substrate;
patterning the silicon layer or the polysilicon layer to form a patterned structure; and
removing the silicon dioxide layer by an aqueous solution of hydrogen fluoride to produce the microstructure. The silicon dioxide layer is used, therefore, as a sacrificial layer. (See "SOI (SIMOX) as a substrate for surface micromachining of single crystalline silicon sensors and actuators") The 7th International Conference on Solid State Sensors and Actuators, Transducers 93, Jun. 7-10, 1993, pp. 233-236).
According to this first process, however, it is necessary to use materials for the micro-structure which are not eroded by hydrogen fluoride, and it is not possible to incorporate an electrode of erodable material, such as an aluminum electrode, in the microstructure.
Furthermore, if polysilicon is adopted as the microstructure material, it is necessary to regulate the internal stress of polysilicon film in order to prevent bending of the substrate.
Furthermore, in the case of using an SOI substrate, buried silicon dioxide is removed. If the removal of this oxide is not carefully controlled, over-etching will occur. It is difficult therefore to maintain contact between the microstructure and the substrate. Also, if aluminum or other metal is deposited after removal of the buried silicon oxide, it is not easy to form a continuous electrode structure due to overhang of the microstructure.
The second process is a process for producing a spatial light modulator device provided with an aluminum micro-mirror. This is described in Japanese Patent Laid-Open Application No. 2-8812. This process comprises coating a photo-resist sacrificial layer on a substrate, then forming an aluminum layer on this sacrificial layer, patterning the aluminum layer, and removing the sacrificial layer by oxygen plasma etching to produce an aluminum film micro-structure.
The micro-structure can be produced on various kinds of substrate and this does not depend on the surface roughness of the substrate.
In addition, since the sacrificial layer is removed by dry etching, here a oxygen plasma etching process, sticking between the substrate and the micro-structure, which can happen when removing the sacrificial layer by a wet etching process, is avoided. However, since it is necessary to deposit the film for the micro-structure at a low temperature to avoid damage to the sacrificial layer, the choice of microstructure material is severely restricted. Furthermore, it is necessary to regulate the internal stress of the film for the microstructure to prevent the microstructure from bending.
The third process is a process in which the pattern for the microstructure is formed on a Si substrate, after which a part of the pattern is bonded anodically to a glass substrate, after which the bonded Si substrate is etched from its back surface until the pattern is left on the glass substrate.
A linear actuator comprising bulk Si film (Y. Gianchandani et al, "Micron-Sized, High Aspect Ratio Bulk Silicon Micromechanical Systems Devices", Proceedings IEEE Electro Mechanical Workshop 1992, pages 208-213), or a cantilever comprising silicon nitride for an Atomic Force Microscope (AFM) may be produced by this process (U.S. Pat. No. 5,221,415).
In this process, it is not necessary to use a sacrificial layer so that micro-structures made of a material which does not have resistance to hydrogen fluoride can be produced.
However, the microstructure materials are limited to those which can be bonded anodically to glass, such as Si, Al, Ti, Ni, which are electroconductive and which can be oxidised, or silicon dioxide film or silicon nitride film coated on a substrate.
Furthermore, when bonding is made anodically a temperature of 300.degree. C. or more is usual and it is necessary therefore to use a glass having the same thermal expansion coefficient as that of the Si substrate to avoid damage to the substrate by heat stress. The choice of glass is limited to pyrex glass (trade name #7740; manufactured by Corning) or the like.
Where an electrode is already provided on the substrate, it is then difficult to produce an electrode on the microstructure.
In addition, it is necessary to use a glass which contains mobile ions as the material of the substrate, such as soda glass, Pyrex and crystallised glass. Consequently, this process is inapplicable to substrates incorporating integrated circuit components.
Furthermore, in the case of bonding the electroconductive material to the glass anodically, it is necessary for the glass and the electro-conductive material to have a surface roughness of 50 nm or less.
In U.S. Pat. No. 5,221,415, silicon nitride is bonded anodically to glass at 475.degree. C. Consequently, electrodes have to be formed by vacuum evaporation on the whole surface of the substrate after producing the microstructure. It is difficult, however, to form a patterned electrode on a beam structure such as a cantilever.